Weight and balance
Money Matters by Keith Sessions, Chief Financial Officer
keithksessions@hotmail.com
Ensuring that an airplane has its center of gravity (CG) in the right place will dramatically impact its overall flight performance. A CG that is outside of the limits will result in an unstable airplane that is hard to control. Minor tweaks of the CG within the limits will make an airplane lock in for precision flight.
Throughout the years, I have seen many ways to determine an aircraft’s CG. The most common is to simply balance it on your fingers or on a couple of rods in a stand. This works great on small models where precision is not that important. Another way is to use a sling, sometimes referred to as a Vanessa rig. This works well with larger models and is more accurate.
The best way to determine and adjust the CG is with a ruler, calculator, and scales. This works well with large airplanes and is the most exact way to determine the CG. In fact, this is how full-scale aviation determines the CG on everything from a J-3 Cub to a 747.
Let’s start by defining a few terms and equations:
Weight: This is self-explanatory, but be certain to use only one unit. In other words, 10.5 pounds or 168 ounces instead of 10 pounds, 8 ounces.
Datum: A point from which all measurements are taken. In full-scale aviation, this is often the firewall. With models, I generally use the tip of the spinner or front of the cowling.
Arm: The distance between the datum and the CG of an item or object.
Moment: Weight x arm
CG: Total moment x total weight
Now, how to make all of this work:
- Lay out a measuring tape under your model on the workbench or the ground.
- Determine your datum point. I put the end of the measuring tape at the datum to make it easier.
- Make sure the airplane is level. This is particularly important for tail wheel aircraft. The tail wheel should be raised to a point where the airplane would fly level. Measuring wing/stabilizer incidence will give you an exact determination. Refer to the plans as to what those values should be.
- Weigh the airplane under each wheel. I use three postage scales that I purchased through Amazon for $15. You can get away with one set of scales, but be sure to keep the airplane level.
- Determine the arm for each wheel as well as the preferred CG location.
- Now use the above formulas to determine the location of the CG.
What happens if the CG is out of range, or you wish to adjust it within the range? There is a simple process for this as well. The formula you will use is:
Total aircraft weight x
distance the CG needs to move
Distance between ballast and desired CG
The number you end up with from this formula is the total ballast needed.
You have to determine where you will put the ballast. This is usually on the firewall or the tail. Take your measurements and apply the formula.
Before adding any weight, try weighing and relocating components needed in the model. Servos might not weigh much, but can make a huge difference if they are located in the tail or close to the CG. The same is true for batteries. I have seen simply relocating the servos make a difference of more than a pound in ballast.
For very detailed information on this subject, refer to the FAA handbook at www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-....
Tools are available that do the calculations for you and include accurate scales. These are great and make the task easier. I was taught long ago how to make calculations with scales, a ruler, and a sheet of paper (I was not allowed to use a calculator). I hope you found this helpful.
On the financial side, AMA passed its internal audit without any issues. As mentioned last year, some numbers were skewed because we had many people take advantage of the two-year membership offer before the rates increased.
We also had an increase in the number of Life Members.
